The invention relates to superconductors in general and more particularly to an improved method for manufacturing a superconductive Nb.sub.3 Sn layer on a niobium surface for high frequency applications through the diffusion of tin into this surface at elevated temperature.
Superconducting devices for operation with high frequency electromagnetic fields at frequencies of about 10 MHz and higher find many engineering applications. They can be employed, in particular, as resonators and separators for particle accelerators or as high frequency resonators for other purposes, e.g., as frequency standards. They may be designed, in this connection particularly, as cavity resonators or as helical resonators. Superconducting cavity resonators are operated in the frequency range of, say, 1 to 15 GHz and superconducting helical resonators in the range around 100 MHz. Niobium and, occasionally, also lead have mostly been used as superconductive materials for such resonators.
In such superconducting devices, a high quality factor and as a rule, also the highest possible critical magnetic flux density B.sub.c.sup.ac, measured under the influence of high frequency fields, so that the superconducting devices can be operated with maximum high frequency power and, at the same time, with low surface resistance is desired. For, if the critical magnetic flux density B.sub.c.sup.ac is exceeded, then the losses rise sharply, the surface resistance increases considerably and the electromagnetic field breaks down. The upper limit for B.sub.c.sup.ac is given by what is known as the thermodynamic critical flux density B.sub.c. Since the thermodynamic critical flux density B.sub.c of Nb.sub.3 Sn is higher than that of niobium, it can be expected that a higher critical flux density B.sub.c.sup.ac can be achieved at an Nb.sub.3 Sn surface than at a niobium surface. In addition, Nb.sub.3 Sn also has a considerably higher critical temperature than niobium, so that it has, for one, a higher thermal stability and, on the other hand, should permit also higher operating temperatures than niobium, particularly for operation at the temperature of boiling liquid helium of 4.2.degree. K, which is already too high for high frequency applications of niobium.
There have been attempts at applying thin protective layers of Nb.sub.3 Sn by first evaporating tin onto the niobium resonator and then heat treating the latter. With such surface layers, a quality factor Q.sub.o of about 10.sup.9 and a critical flux density B.sub.c.sup.ac of about 25 mT have been measured at 2.8 GHz (cf., "Siemens- Forschungs- and Entwicklungsberichte" 3 (1974), page 96, righthand column).
In addition, it is known to expose the niobium parts which are to be provided with an Nb.sub.3 Sn layer, in a closed reaction vessel, i.e., a closed off, evacuated quartz ampoule, at an elevated temperature of about 1000.degree. C to a tin vapor atmosphere, from which the tin diffuses into the niobium surface, forming the desired Nb.sub.3 Sn layer. With this method, Nb.sub.3 Sn layers of several micrometers thickness and with already relatively good properties, e.g., quality factors Q.sub.o of about 10.sup.9 and critical magnetic flux densities B.sub.c.sup.ac of somewhat above 40mT at 1.5 K, can be obtained (paper by Hillenbrand et al in "IEEE Transactions on Magnetics," vol. MAG-11, No. 2, March 1975, pages 420 to 422).